Why can't we use tazorac or retin a?

Does anyone know why specifically. I am a little concerned that my doctor didn't warn me. I've been using this for a long long time and now I'm pregnant. Should I be worried?

From Babycenter.com

These powerful substances, found in some antiaging moisturizers, are lauded for helping reduce wrinkles and improve skin tone. Retinoids are a type of vitamin A that speeds up cell division (quickening your skin's renewal) and prevent skin collagen from breaking down.

But retinoids are one of the skin-care ingredients that experts, including Baumann, recommend that expectant moms stay away from. Some studies have shown that high doses of vitamin A during pregnancy can be harmful to an unborn child. And oral retinoids, such as isotretinoin (Accutane, an acne treatment), are known to cause birth defects.

If you've been using a skin cream that contains a retinoid, don't panic. Retinoids have not been shown to cause problems in their topical form in pregnant women.

"There is no data to show these retinoids ingredients are harmful when used on the skin — doctors are just being extra cautious," explains Baumann.

On the label:
Differin (adapelene)
Retin-A, Renova (tretinoin)
Retinoic acid
Retinol
Retinyl linoleate
Retinyl palmitate
Tazorac and avage (Tazarotene)

Bottom line:
Best to avoid